Japanese published Patent Application (OPI) Nos. 102827/74 and 42025/75 (the symbol OPI as used herein means an unexamined published Japanese patent application) disclose processes for producing a slow release agent wherein a physiologically active substance is supported on protein from various protein sources such as collagen and gelatin. But the efficacy of the slow release agents prepared by such conventional methods lasts for only several hours to ten-odd days. In this respect, none of the slow release agents developed so far make the most of protein as a substance compatible with the living tissue.